This invention relates to spread spectrum signal processing and in particular to a method and apparatus for detecting zero-crossings of a pseudo-noise (PN) spread spectrum modulated signal which is translated to baseband by sampling an intermediate frequency (IF) signal.
A spread spectrum system as described in U.S. Pat. No. 4,550,414, issued Oct. 29, 1985 to Walter J. Guinon and Richard H. Lamb, Jr. and assigned to the Charles Stark Draper Laboratory, Inc., is one in which the transmitted signal is spread over a wide frequency band, much wider in fact than the minimum bandwidth required to transmit the information being sent. The following are three general types of modulation which produce spread spectrum signals: (1) Modulation of a carrier by a digital code sequence; such systems are called direct sequence modulated systems; (2) Carrier frequency shifting in discrete increments in a pattern dictated by a code sequence; these are called frequency hoppers because the transmitter jumps from frequency to frequency; and (3) Pulsed-FM or "chirp" modulation in which a carrier is swept over a wide band during a given pulse interval.
The term pseudo-noise (PN) refers to a predetermined apparently random pulse sequence having a recurring period or cycle which is long compared with a prevailing information or message duration. This pseudo-random pulse sequence is usually used in a direct sequence system which, in a binary phase shift keyed (BPSK) embodiment, involves a carrier which is 180.degree. phase-shifted in accordance with the output of a pseudo-random-number code generator. Thus, the incoming signal consists of a sequence of phase transitions from one constant value to another.
More and more in communications and radar the modulated carrier signals depend entirely on when the signal crosses zero, rather than on its amplitude. Radar and communication signals are spread across the frequency spectrum to better resist interference and jamming instead of being transmitted at one frequency. Pseudo-noise (PN) spread spectrum modulation is employed in communications systems for purposes of security, anti-jam protection, multiple access capability or a safe level of power flux density radiated back to earth. When the interference and jamming signals are real strong, adaptive techniques are used to suppress such interference and jamming signals.
In the prior art, baseband sampling has been performed using dual A/D's, one for an I data channel and another for a Q data channel in addition to threshold control logic circuits as described in a paper by F. Amorasa titled "Adaptive A/D Converter to Suppress CW Interference in Spread-Spectrum Communications," IEEE transactions on Communications, Vol. Com-31, pp. 1117-1123, October 1983. This approach requires extra hardware for I and Q baseband mixing and analog to digital converters. The dual mixing and converters have bias problems due to gain and phase differences in each channel, and baseband adaptive techniques are more difficult to control making such a baseband system easier to jam.